Method and apparatus for monitoring wall thickness of blow-molded plastic containers

ABSTRACT

A method of inspecting wall thickness of blow-molded plastic containers includes providing a blow-molder having a plurality of molds and a plurality of associated spindles. The containers are inspected by impinging infrared light thereon and detecting the portion of the infrared light that passes through the container and converting the same into corresponding electrical signals which are delivered to a microprocessor. The microprocessor compares the electrical signals with stored information regarding desired wall thickness of the container and emits output thickness information. The microprocessor receives signals from sensors associated with the blow-molder relating to the position of the molds, the identity of the molds and the identity of the spindles in order that the wall thickness information that is determined by inspection can be associated with specific molds and spindles. The wall thickness information may be averaged over a period of time or a number of containers. The information may be visually displayed so as to provide feedback regarding the inspection process. In a preferred embodiment wall thickness measurements are taken at several vertically spaced positions simultaneously. Corresponding apparatus is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention provides an automated on-line method and apparatus for inspecting wall thickness of blow-molded plastic containers and, more specifically, it provides feedback regarding the thickness measurements such that the particular container being inspected is associated with a specific mold and spindle.

2. Description of the Prior Art

It has long been known that plastic containers such as bottles must be inspected in order to make sure that the wall thickness is adequate for the desired purpose and that the use of excess material is minimized.

In general, it has been known to employ sampling inspection techniques wherein, at a periodic intervals, which might be on the order of once per hour, a container was removed from the conveyance system after the container emerged from the blow-molder and was destructively tested by cutting the same into multiple, horizontal sections which were then weighed with the weight being correlated with the wall thickness.

An alternative inspection method involved measuring the wall thickness of such containers by nondestructively testing sample plastic containers. A suitable system for effecting such testing is the AGR Top Wave Profiler Gauge PG 9800. A suitable laboratory instrument for this latter approach is that sold under the trademark AGR Top Wave Wall Thickness Profiler. One of the problems with such an inspection approach is that it was time-consuming and labor intensive. Also, the long interval between samplings resulted in a delay in process feedback which in turn could result in reduced production efficiencies.

It has also been known to employ high-speed on-line wall thickness monitoring systems for blow-molded plastic containers. These systems provide real-time monitoring of material distributions and rejection of defects. A suitable system for such purpose is that sold under the trademark AGR Top Wave PET Wall System. While these systems represent a substantial improvement in the completeness of sampling by inspecting each container and the timing of same, they did not provide feedback coordinated with the operation of the blow-molding machine.

U.S. Pat. No. 4,304,995 discloses a system for measuring wall thickness of plastic containers employing infrared absorption. The containers are sampled off-line and required the use of rotation and disclosed the use of radiation sources and radiation detectors which were structured to rotate with respect to each other.

U.S. Pat. No. 4,490,612 discloses a method of measuring the thickness of plastic film using relative absorptions of two infrared wavelengths.

U.S. Pat. No. 5,139,406 discloses the use of infrared absorption in measuring the wall thickness of plastic containers. On-line measurement is contemplated, but this system requires insertion of a probe into the container. Such an approach is uneconomical and inefficient in respect of current blow-molder plastic container production speeds.

U.S. Pat. No. 5,591,462 discloses the use of machine vision technology in monitoring certain defects in blow-molded containers. Among the features being monitored by this system are seal surface, base and neck folds and finish gauge inspection.

PCT publication WO 01/65204 discloses a method and apparatus for measuring plastic containers on-line employing infrared absorption. The apparatus was said to be employable on a conveyer or inside the blow-molder. It made use of laterally homogenous material distribution properties and measured though both sides of the container.

In spite of the foregoing prior art disclosures, there remains a very real and substantial need for an improved inspection system for blow molded plastic containers which will provide timely and accurate feedback regarding not only whether a container fell within the wall thickness specifications, but also identity of the molds and associated spindles which produced the container.

SUMMARY OF THE INVENTION

The present invention has met the above-described need.

The method of the present inventions involves inspection of the wall thickness of blow-molded plastic containers by providing a plastic container blow-molder having a plurality of molds and a plurality of associated spindles. The containers are inspected by impinging infrared light thereon and detecting the portion of the infrared light that passes though the container and converting the same to corresponding electrical signals which are delivered to a microprocessor. The microprocessor receives the thickness related signals and compares them with stored information regarding the desired thickness and emits thickness information. A visual display of such information, which may include an average container wall thickness over a period of time, for each mold and spindle may be provided.

The method may involve providing a plurality of such systems so that container wall thickness may be measured substantially simultaneously at a plurality of elevations.

The method includes sensing a plurality of conditions in the blow-molder, including mold position, mold identity and spindle identity such that the thickness determined can be synchronized with a particular mold and spindle to thereby provide meaningful feedback regarding the thickness determination.

The presence of a container to be inspected in the inspection station is also provided. A reject mechanism for physically removing a rejected container is also provided.

The apparatus of the present invention includes an inspection station preferably disposed inside of the blow-molder and having at least one source of infrared radiation which impinges the radiation on the plastic container to be inspected and cooperating photodetectors which may be photoconductive lead-sulfide infrared detectors, for example.

These receive the infrared radiation passing through the container and convert the same into corresponding electrical signals which are delivered to the microprocessor. The microprocessor contains stored information regarding the desired thickness and is structured to effect a comparison and issue thickness information output signals which may go to a visual display unit for presentation to an operator and may also, if the container is to be rejected, present such a signal to the reject mechanism which will remove the container from the line. Sensors for sensing the mold assembly position, as well as the identity of each mold and spindle so as to synchronize the same with the container being inspected are provided and are preferably disposed within the blow-molder.

It is an object of the present invention to provide an improved automated on-line rapid inspection system for inspecting wall thickness of plastic containers, such as bottles, for example.

It is another object of the invention to provide a method an apparatus for effecting such inspection while providing meaningful feedback regarding the specific mold and spindle which made a given container.

It is another object of the present invention to provide such a system which employs sensors within the blow-molder to provide information to a microprocessor regarding mold position and mold and spindle identity as related to a specific blow-molded container.

It is an object of the present invention to provide a system which is adapted for rapid on-line assembly of plastic bottles and other plastic containers made by blow-molding in such a manner as to identify the mold and spindle which made a specific container.

It is a further object of the present invention to provide such a system which will facilitate immediate communication of wall thickness information for either manual or automated control of the blow-molding system.

It is a further object of the invention to provide such a system which enhances the efficiency of the manufacture of blow-molded plastic containers.

These and other objects of the present invention will be more fully understood from the following description of the invention on reference to the illustrations appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing the blow-molder, container transporting mechanisms, the inspection area and reject area.

FIG. 2 is a perspective view showing a form of light source and associated photodetector employable in the inspection station of the present invention.

FIG. 3 is a schematic diagram showing a form of apparatus usable in the present invention and the interaction of the same.

FIG. 4 is an algorithm flow chart illustrating the flow of information in an embodiment of the present invention.

FIGS. 5(a), (b) and (c) illustrate a timing diagram showing the relationship among machine step, mold sync and spindle sync signals.

FIG. 6 illustrates a screen of a visual display unit employable in the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As employed herein, the term “containers” refers to plastic bottles, jars, vials and other plastic containers usable for storage of liquid and other flowable materials. Examples of the size of containers for which the present invention is particularly well suited are containers having a capacity of about 0.2 to 3 liters.

In a typical prior art plastic container, blow-molding process preforms entering the blow-molder are typically at room temperature. The preforms are inverted and loaded, upside-down, onto spindles. The spindles carry the preforms through the reheat oven which raises the temperature of the plastic in preparation for blow-molding. Uniform heating is important so the spindles rotate as they traverse through the oven. There are typically 100 to 400 spindles, forming a conveyor loop. After exiting the reheat oven, the preforms are removed from the spindles and transferred by a system of transfer wheels into the molds on the mold wheel. Failure of the spindles to rotate correctly while traversing through the oven will result in a poor thickness distribution in the blown container.

Employing one or more light sources of infrared radiation and cooperating associated photodetectors preferably located within the blow-molder near the output portion of a blow-molder where the containers are extracted from the molds, container wall thickness can readily and rapidly be determined. It is known that plastic materials such as PET absorb infrared radiation of specific wavelengths. This facilitates determination of the thickness of the container wall based on the amount of infrared radiation that has been absorbed. In a preferred practice of the present invention, the thickness monitoring apparatus and method will employ two distinct infrared wavelengths in order to compensate for refractive and scattering effects that might otherwise have a deleterious effect on the measurement.

Referring to FIG. 1, a preform oven 2 typically carries the plastic preforms on spindles through the oven section so as to preheat the preforms prior to blow-molding of the containers. The preforms leaving the preform oven 2 enter the mold assembly 6 which contains a plurality of molds by means of conventional transfer apparatus 7 (shown in phantom). The blow-molder 4, which may be of conventional type, has the array of molds which may be on the order of ten to twenty-four arranged in a circle and rotating in a direction indicated by the arrow C. Containers emerging from the mold assembly 6, such as container 8, will be suspended from a transfer arm, such as 10, on transfer assembly 12 which is rotating in the direction indicated by arrow D. Similarly, transfer arms 14 and 16 will, as the transfer assembly 12 rotates, pick up a container such as 8 and transport it through the inspection area 20 which will be described in greater detail hereinafter. A reject area 24 has a reject mechanism 26 which will physically remove from the transfer assembly 12 any containers deemed to be rejects. Container 30 has passed beyond the reject area 24 and will be picked up in star wheel 34 which is rotating in direction E and has a plurality of pockets, such as 36, 38, 40, for example. Container 46 is shown as being present in such a star wheel pocket. The containers will then be transferred in a manner known to those skilled in the art to conveyer means according to the desired transport path and nature of the system.

Referring to FIG. 2, there is shown a form of inspection station 20 which has a container 60 passing therethrough in the direction indicated by the arrow under the influence of a suitable conveyance device (not shown). In the form shown, a plurality of >light sources 64, 66, 68 are vertically spaced from each other in order to inspect the wall thickness of the bottle at three zones at three different elevations. Cooperating with the light sources 64, 66, 68, respectively, are photodetectors 74, 76, 78. In operation, infrared radiation will be emitted by the light sources 64, 66 68, impinge upon bottle 60, have a portion of the infrared radiation absorbed by the plastic container 60 and have the remaining infrared radiation impinge upon the detectors 74, 76, 78 which will convert the received light into a corresponding electrical signal which will be delivered to a microprocessor for further processing. Any suitable detector which will function efficiently with the infrared radiation wavelengths employed may be used. A preferred detector is a photoconductive lead-sulfide (PbS) infrared detector. A suitable PbS detector is that sold by CalSensors. In a preferred system, the detector assembly consists of a prism-grating-prism spectrograph and two or more PbS detectors (such an assembly is manufactured by Spectral Imaging, Ltd. Of Finland, using PbS detectors from CalSensors). The spectrograph disperses the infrared radiation as a function of wavelength; the detectors are located so as to be sensitive to specific wavelengths of infrared radiation. One wavelength is selected to correspond to an absorption band in the plastic container. A second wavelength is selected to correspond to a transmission band in order to provide a reference. As an alternative to using the spectrograph, band-pass optical filters may be used in conjunction with the PbS detectors.

Referring still to FIG. 2, further details regarding the creation of synchronized wall thickness determination as related to specific molds and spindles will be considered. The light source preferably includes a halogen bulb that is always on, a lens to collect and collimate the light into a beam, a spinning segmented disk that “chops” the light beam and a remotely-controlled calibration disk. The light source is preferably always “on,” emitting a pulsed beam (which preferably pulses at about 600 Hz). The light source emits a pulsed beam of “white” light, containing all of the desired infrared wavelengths.

Referring to FIG. 3, there is shown a microprocessor 90 which, in the form shown, exercises control over the calibration disks, which are preferably integral with light sources 92, 94, 96 and 98. A container which will pass through the gap indicated generally as 100 will, in the form shown, receive light from sources 92, 94, 96 98, absorb a portion of the same and then have the light not absorbed impinge on photodetector sensors 102, 104, 106 108, respectively, which will convert the received light into corresponding electrical signals which are delivered to the microprocessor 90.

In a preferred embodiment of the invention, three key sensors which are within or operatively associated with the blow molder, provide information to enable synchronization of the specific molds and spindles which made the container being inspected and thereby provide valuable feedback information. One sensor, designated the blow-molder machine step sensor 120, emits a signal which contains information regarding the counting of the molds and spindles from their corresponding starting position. The total number of molds or spindles may vary depending upon the make and model of blow-molder, but this information is known in advance. This information may be programmed into the system. A second signal, which is from the blow-molder synchronization sensor 122, provides information regarding start of a new cycle of rotating the mold assembly. The output of this sensor 122 is provided to microprocessor 90. The blow-molder spindle synchronizing sensor 126 provides output regarding the new cycle of rotating the spindle assembly. This output is provided to the microprocessor 90. The sensors employed for monitoring machine step mold sync and spindle sync may be positioned at any suitable location within the blow-molder and may be of any suitable type, such as inductive sensors which are well known to those skilled in the art.

The part-in-place sensor 130 provides a signal to the computer indicating that a container has arrived at the inspection station and that the wall thickness inspection should be initiated. At that point, the container transects the beams of white light containing all of the desired infrared wavelengths emitted by light sources 92, 94, 96, 98. The system preferably employs an incandescent light bulb that is operated in a continuous mode. This continuous light is preferably mechanically shuttered at the desired 600 Hz by a rotating segmented disk contained in the light source assembly. The output of the light source is a pulsed beam of light. This pulsed radiation is designed to match the characteristics of the detectors. The microprocessor 90 receives the electrical signal and effects a comparison of the thickness information contained within the electrical signal with stored information regarding desired thickness. If the thickness is not within the desired range, it emits a signal to the blow-molder reject 140 which in turn initiates a rejection signal to operate rejection apparatus 24, 26 (FIG. 1) and discard that container from the conveyer. The output thickness information from the microprocessor 90 will be delivered to touchscreen display 150 which provides an operator with information regarding specific containers produced by particular mold and spindle combinations. It is preferred that the values be averaged over a period of time which may be on the order of 30 seconds to ten minutes. In addition or in lieu of time measurement, the average may be obtained for a fixed number of containers which may be on the order of 2 to 2500. The operator also obtains trend information for the blow-molder and individual molds and spindles through the visual display unit 150. In the event of serious problems requiring immediate attention, visual and/or audio alarms may be, provided. As indicated by the dual arrows F and G, an operator may input certain information to the microprocessor 90 to alter calibrations in order to control operation of the microprocessor. The operator may input process limits and reject limits into the microprocessor 90 for each of the thickness measurement zones. The reject limits are the upper and lower thickness values that would trigger the rejection of a container. The process limits are the upper and lower values for the time-averaged or number of container averaged thickness that would trigger a process alarm indicator. Also, if desired, hard copy or other output of the microprocessor 90 results may be provided as by output 152 which may be a conventional printer, for example.

The microprocessor 90 display highlights molds or spindles having undesirable thickness—either too thick or too thin. For example, if one mold was producing containers that are too thick or too thin, the operator would adjust mold-related parameters such as blow-pressure or blow-rate to correct the problem; or the operator might need to stop the blow-molder to replace or repair an air valve for that mold. It will be appreciated that the mold/spindle-correlated feedback provided by the microprocessor is used to localize the problem.

Referring to FIG. 4, an algorithm flow chart showing the method of the inspection process, container tracking and combining mold and spindle information of the present invention will be considered. As indicated in FIG. 3, the blow-molder machine step sensor 120 will provide an output identified in FIG. 4 as 180 and the blow-molder mold sync sensor 122 will provide an output signal 182 and the blow-molder spindle sync sensor 126 will provide a spindle sync signal 186. As shown by block 190, the machine step signal 180 contains information regarding the incremental movement of the mold module, the number of molds and the incremental spindle module and the number of spindles. The mold sync signal 182 will verify that the mold is equal to the mold offset with resetting being accomplished if necessary.

In order to adjust for the fact that microprocessor 90 may start up in the middle of a blow-molding cycle, the microprocessor 90 preferably employs an algorithm that allows the microprocessor to re-synchronize with the blow-molder 4 within one mold or spindle cycle. The microprocessor 90 then remains synchronized with the blow-molder 4. The algorithm is:

Machine-step event: increment mold#, if mold# is greater than number of molds, reset to 1 increment spindle#, if spindle# is greater than number-of-spindles, reset to 1 Mold-sync event: Set mold# to X (mold offset as pre-configured) Spindle-sync event: Set spindle# to Y (spindle offset as pre-configured)

Similarly, the spindle sync signal 186 will verify that the spindle equals the spindle offset with a reset being achieved, if necessary. The collective output of blocks 190, 192 and 194 is detailed information with respect to the current mold and spindle identity and position with respect to the container being inspected. The sensor 130 (FIG. 3), when a container has reached the inspection level will emit signal 210 which is combined in block 212 by associating the specific mold and signal with this particular container and this container is tracked in synchrony with the specific mold and spindle. In the next process block 214, the microprocessor will collect and process the infrared sensor data, calculate the thickness and merge the results with the corresponding container in the tracking queue.

The output of block 214 proceeds to block 216 where, if the container is being rejected, it is tracked to the rejection point and a decision regarding pass and reject has been made.

Finally, the microprocessor in block 218 updates the container thickness trend database and communicates the thickness information to touchscreen display 150 (FIG. 3). This ends the tracking of that container. It will be appreciated that the net result is that the particular container being inspected is associated with a particular mold and associated spindle with a reject or pass decision determining whether the particular container remains in the conveying process or is excluded by the reject mechanism. The information also serves to update the thickness trend database as displayed in unit 150 and printed or otherwise stored or processed in output unit 152 (FIG. 3).

Referring to FIG. 5, there is shown in FIG. 5(A) the machine step timing diagram with there being a one-for-one correspondence between the machine step pulses =>and containers produced by the blow-molder. The mold sync pulse shown in FIG. 5(B) indicates the start of a new cycle of the mold wheel assembly and the spindle-sync pulse as shown in FIG. 5(C) shows the start of a new cycle of the spindle loop.

At the inspection station, there is a fixed phase relationship between the mold sync pulse and the machine step pulse corresponding to the first mold. This phase information, which may be referred to as the “MoldOffset,” is determined when the system is installed into the blow-molder and then is entered into the processor. Similarly, the SpindleOffset is determined during installation and entered into the process.

Referring to FIG. 6, there is shown a visual display screen 240 which could be presented on the touchscreen display unit 150 (FIG. 3) to provide prompt and concise feedback regarding the mold/spindle correlated thickness information for purposes of process control and blow-molder optimization. The process status is shown in FIG. 6. The representation on the left shows a container 250 which in the form shown is a bottle having an exteriorly threaded neck. The wall thickness has been measured at vertically spaced levels 252, 254, 256. Each band 252, 254, 256 will contain a numerical indication of the average wall thickness. These indicated numbers show the process-wide average thickness at these measurement locations averaged over a certain selected period of time which may be on the order of 30 seconds to 10 minutes or could be an average of a number of containers from about 2 to 2500.

Referring still to FIG. 6, it is noted by way of example that band 252 is subdivided into a plurality of units 255, 257, 259, 260, 262, each of which may be presented in a distinctive color different from next adjacent subportions of band 252 for ease of visual review. By way of example, the numbers underlying band 252 present a scale of thickness in inches taken to four decimal points. Overlying the band 252 appears the number 0.2088 with an inverted triangle pointing to a portion of band 252. This number represents an average wall thickness at that location of the bottle based on, for example, a period of time or a number of containers measured. One seeing the computer screen 240, therefore, can quickly ascertain not only quantitatively what the average thickness measurement has been, but also visually in terms of the position on the scale. Similar numerical scales and reading information would preferably be contained on bands 254, 256.

On the right in FIG. 6 is a graphic representation of the mold wheel assembly 280 having each mold represented by a circle and containing information regarding the related container thickness. In the center of the mold circle, there is a grid 300 showing container thickness status for a number of spindles. As the number of spindles can be quite large, the display shows a pareto-optimized list of problem spindles with the identity of the worst spindle problems being identified by a spindle number or other identifier.

With respect to the molds, it is noted that some indication regarding thickness may be provided by the use of different colors. For example, as shown, the number 290 points to a mold which has a white representation, as does 292. The remaining molds are shown in black. A suitable scale may be provided so that the white indicates a thickness above or below control limits and the black indicates a thickness within limits. As these circles may contain numbers (not shown) identifying a particular mold, this will enable an operator to obtain a visual indication regarding the average thickness as related to control limits or reject limits for that mold. With regard to spindle representing grid 300, as there are more spindles than shown in the grid, this embodiment would employ the worst of the spindles in respect of containers which have been inspected and having the greatest departure from desired wall thickness. By way of example, the top row of squares identified respectively by reference numbers 304, 306, 308, 310, 312, 314 are identified respectively and related to spindles 1, 3, 12, 20, 21, 23. As is true with the molds, these grid representations would preferably have color coding indicating as to each spindle in the grouping, the degree of departure from the control limits or reject limits or, in the event that it is within limits, a color indicating that category. It will be appreciated that while the drawings show color representations for the molds as being black or white, and no color distinctions are provided in the illustrated grid 300, two or more colors may be employed in respective circles and blocks to indicate various thickness averages as related to the desired limits.

If desired, additional information may be provided on the screen 240. For example, if the average is based upon a time of 3 minutes, a legend to that effect may be provided. Similarly, if the average thickness is based upon the last 250 bottles, a legend to this effect may be provided. Also, information regarding the total number of rejects and the percentage of rejects may be provided. Numerical indications of the number of rejects coming from each of the molds and spindles may also be provided. The color codes or symbols such as “+” or “−” may be employed to identify whether the departure from desired control limits or reject limits are above or below such limits.

Where two distinct wavelengths of infrared radiation are used, a first will be at a wavelength which is readily absorbed by the plastic material of the container and the other wavelength will be only slightly absorbed. A further possibility is that the containers may be filled with condensed water vapor at the end of the blow-molding process. If that is sufficiently dense, the internal fog formed in the container may scatter light away from the sensors and interfere with measurement. If desired, a third infrared wavelength which is not at an absorption band with respect to the plastic material can be used in order to calculate a correction factor to enhance the accuracy of the thickness measurement by correcting for optical scattering caused by the fog.

It will be appreciated that the present invention has provided an improved automated system for wall thickness determination in a plastic container which, as a result of sensors operatively associated with the blow-molder, provides detailed information so as to correlate wall thickness of a given container with the mold and spindle at which it is made. The microprocessor processes data regarding the thickness measurement and outputs the same to a unit which may visually display and/or to another unit which may provide hard copy of the average thickness readings which may also be a thickness reading achieved over a period of time such as about 30 seconds to 10 minutes or a number of containers which may be about 2 to 2500.

Whereas, particular embodiments of the invention have been described here for purposes of illustration, it will be evident to those skilled in the art that numerous variations of the details may be made without departing from the invention as set forth in the appended claims. 

1. A method of inspecting wall thickness of blow-molded plastic containers comprising providing a plastic container blow-molder having a plurality of molds and a plurality of spindles, inspecting said container by impinging infrared light thereon from the exterior thereof and detecting the portion of said infrared light that passes through two walls of said container, converting said detected light into corresponding electrical signals which are delivered to a microprocessor, comparing in said microprocessor said electrical signals with stored data regarding desired wall thickness and emitting output thickness information regarding said container wall thickness, and delivering to said microprocessor signals received from sensors associated with said blow-molder relating to said mold and said spindle involved in making the container being inspected.
 2. The method of claim 1, including said microprocessor output thickness information having identification of said mold and said spindle employed in making said container and employing two distinct infrared wavelengths in said inspection method.
 3. The method of claim 2, including delivering to said microprocessor signals from a sensor associated with said blow molder for determining the position of said molds and said spindles.
 4. The method of claim 2, including automatically rejecting any container which does not pass said inspection by separating the same from adjacent non-rejected containers.
 5. The method of claim 2, including sensing within said blow-molder the position of said molds.
 6. The method of claim 5, including delivering said sensor obtained mold position information to said microprocessor.
 7. The method of claim 2, including said sensors associated with said blow-molder being disposed within said blow-molder.
 8. The method of claim 2, including employing said output thickness information to adjust operation of said blow-molder when undesired thickness variations occur.
 9. The method of claim 2, including employing said output thickness information to initiate an alarm when predetermined conditions exist.
 10. The method of claim 2, including inspecting a plurality of vertically spaced portions of said container substantially simultaneously.
 11. The method of claim 10, including said microprocessor providing output thickness information for each said vertically disposed portion.
 12. The method of claim 11, including employing a plurality of detectors in effecting said detecting, and employing a plurality of infrared light sources each operatively associated with a said detector.
 13. A method of inspecting wall thickness of blow-molded plastic containers comprising providing a plastic container blow-molder having a plurality of molds and plurality of spindles, inspecting said container by impinging infrared light thereon and detecting the portion of said infrared light that passes through said container, converting said detected light into corresponding electrical signals which are delivered to a microprocessor, comparing in said microprocessor said electrical signals with stored data regarding desired wall thickness and emitting output thickness information regarding said container wall thickness, delivering to said microprocessor signals received from sensors associated with said blow-molder relating to said mold and said spindle involved in making the container being inspected, and employing two distinct infrared wavelengths in said inspection method.
 14. The method of claim 12, including providing a reject station for physically removing rejected containers, and effecting said inspection after said container emerges from said blow-molder but prior to the container's reaching said rejection station.
 15. The method of claim 13, including effecting said inspection on-line.
 16. The method of claim 13, including prior to initiating an inspection cycle confirming the presence of a container at the desired inspection position.
 17. The method of claim 13, including employing a first said infrared wavelength which corresponds to an absorption band of said plastic, and employing a second said infrared wavelength which is only slightly absorbed by said plastic.
 18. A method of inspecting wall thickness of blow-molded plastic containers comprising providing a plastic container blow-molder having a plurality of molds and a plurality of spindles, inspecting said container by impinging infrared light thereon and detecting the portion of said infrared light that passes through container, converting said detected light into corresponding electrical signals which are delivered to a microprocessor, comparing in said microprocessor said electrical signals with stored data regarding desired wall thickness and emitting output thickness information regarding said container wall thickness, delivering to said microprocessor signals received from sensors associated with said blow-molder relating to said mold and said spindle involved in making the container being inspected, inspecting a plurality of vertically spaced portions of said container substantially simultaneously, said microprocessor providing output thickness information for each said vertically disposed portion, and visually displaying at least a portion of said microprocessor emitted output thickness information.
 19. The method of claim 18, including in said output thickness information associating said container thickness information with identification of the mold and the spindle involved in the manufacture of said container.
 20. The method of claim 19, including in said output thickness information a thickness averaged for a time period.
 21. The method of claim 20, including updating said output thickness information after each inspection of a said container.
 22. The method of claim 20, including said time period being about 30 seconds to 10 minutes.
 23. The method of claim 19, including including in said output thickness information a thickness averaged for a number of containers.
 24. The method of claim 23, including said number of containers being about 2 to
 2500. 25. Apparatus for inspecting wall thickness of blow-molded plastic containers comprising a blow-molder having a plurality of molds and a plurality of spindles, at least one source of infrared light for impinging said light on said container from the exterior thereof, at least one infrared detector for receiving infrared light passing through two walls of said container and converting the same into corresponding electrical signals, a microprocessor for receiving said electrical signals and comparing the same with stored data and determining the wall thickness of said container and whether it is within desired limits and emitting output thickness information, and blow-molder sensors preferably associated with said blow-molder to identify said molds and spindles and provide input to said microprocessor from which it can associate said container thickness information with the mold and spindle which made said container.
 26. The apparatus of claim 25, including a machine sensor associated with said blow-molder for providing information regarding the positions of said mold and delivering a machine position signal to said microprocessor, and said source of infrared light emitting infrared of two different wave lengths.
 27. The apparatus of claim 26, including a container position sensor for confirming that a container is present at the desired inspection position and emitting a signal to said microprocessor.
 28. The apparatus of claim 26, including said microprocessor thickness information including average container wall thickness over a predetermined time period.
 29. The apparatus of claim 28, including said predetermined time period being about 30 seconds to 10 minutes.
 30. The apparatus of claim 25, including a plurality of said infrared light sensors and infrared detectors to permit inspection of said wall thickness at a plurality of vertically spaced locations, a visual display unit for receiving output thickness information from said microprocessor and displaying the same, and said visual display unit providing separate information for each said mold and each said spindle.
 31. The apparatus of claim 30, including said visual display unit displaying information regarding average wall thickness over a period of time.
 32. The apparatus of claim 30, including reject apparatus for removing a container if said microprocessor determines that the wall thickness is not within a desired range.
 33. The apparatus of claim 30, including said microprocessor being structured to update said thickness information after each inspection of a said container.
 34. The apparatus of claim 30, including said microprocessor thickness information being an average thickness, and said visual display unit displaying said average thickness as related to specific said molds and spindles.
 35. Apparatus for inspecting wall thickness of blow-molded plastic containers comprising a blow-molder having a plurality of molds and a plurality of spindles, at least one source of infrared light for impinging said light on said container, at least one infrared detector for receiving infrared light passing through said container and converting the same into corresponding electrical signals, a microprocessor for receiving said electrical signals and comparing the same with stored data and determining the wall thickness of said container and whether it is within desired limits and emitting output thickness information, blow-molder sensors preferably associated with said blow-molder to identify said molds and spindles and provide input to said microprocessor from which it can associate said container thickness information with the mold and spindle which made said container, and said source of infrared light emitting infrared light of two different wave lengths.
 36. The apparatus of claim 35, including said apparatus being positioned for on-line inspection of said containers.
 37. The apparatus of claim 35, including said microprocessor thickness information including average container wall thickness for a number of containers.
 38. The apparatus of claim 37, including said number of containers being about 2 to
 2500. 39. The apparatus of claim 34, including said microprocessor average thickness information being for a specific period of time.
 40. The apparatus of claim 34, including said microprocessor average thickness information being for a specific number of containers. 